This invention relates to the co-production of aromatics, especially C.sub.6 -C.sub.8 aromatics, and olefins, especially C.sub.2 -C.sub.4 olefins, from paraffinic feedstocks (e.g. Udex raffinate) by converting these feedstocks in the presence of a medium-pore zeolite catalyst having closely controlled acid activity.
U.S. Pat. No. 3,756,942 to Cattanach, incorporated by reference as if set forth at length herein, discloses a process for converting paraffinic feedstocks over medium-pore zeolites to produce a variety of upgraded hydrocarbon products. The underlying chemistry involved in this conversion is extremely complex, including cracking of paraffins, aromatization of olefins, and alkylation and dealkylation of aromatics. The article "M2 Forming-A Process for Aromatization of Light Hydrocarbons", by N. Y. Chen and T. Y. Yan, 25 Ind. Eng. Chem. Process Des. Dev. 151, 1986 provides a general overview of the reactions and mechanisms believed to be involved in such aromatization reactions. Products from the conversion of C.sub.5 +paraffinic feedstocks over medium-pore zeolites such as ZSM-5 include C.sub.6 -C.sub.8 aromatics, C.sub.2 -C.sub.4 olefins, C.sub.9 +aromatics and C.sub.1 -C.sub.3 paraffins. Of these products the C.sub.6 -C.sub.8 aromatics and C.sub.2 -C.sub.4 olefins are most desired.
C.sub.6 -C.sub.8 aromatics, e.g. benzene, toluene, xylene and ethylbenzene, also known collectively as BTX, are valuable organic chemicals, useful both as intermediate feedstocks as well as saleable end products. Since BTX has a high octane value it can be used as a blending stock for making high octane gasoline. In contrast, C.sub.9 +aromatics (i.e. aromatic compounds having at least 9 carbon atoms) tend to have a relatively low octane value.
C.sub.2 -C.sub.4 olefins, e.g. ethylene, propylene and butene, are also valuable organic chemicals which can be used to form polymers. By way of contrast, C.sub.1 -C.sub.3 paraffins (i.e. methane, ethane and propane), particularly in admixture, are less valuable chemicals which are generally used for fuel.
From the foregoing, it can therefore well be seen that it would be highly desirable to shift selectivity in a process for upgrading paraffinic feedstreams toward more valuable products including C.sub.6 -C.sub.8 aromatics and C.sub.2 -C.sub.4 olefins.
The acid catalytic activity of zeolite catalysts, for example, aluminosilicate ZSM-5, is proportional to aluminum content in the framework of the zeolite. The more aluminum in the zeolite frmework, the greater the acid catalytic activity of the zeolite, particularly as measured by alpha value. Note the article by Haag et al., "The Active Site of Acidic Aluminosilicate Catalysts," 309 Nature, 589-591 (1985), especially FIG. 2 on page 590 thereof. Medium-pore zeolites with very little framework aluminum and correspondingly low acid catalytic activity can be prepared from reaction mixtures containing sources of silica and alumina, as well as various organic directing agents. For example, the Dwyer et al. U.S. Pat. No. 3,941,871, the entire disclosure of which is expressly incorporated herein by reference, describes the preparation of ZSM-5 from a reaction mixture comprising silica, tetrapropylammonium ions and no intentionally added alumina. The alumina to silica molar ratio of the ZSM-5 produced by this method may be less than 0.005.
U.S. Pat. No. 4,341,748, the entire disclosure of which is expressly incorporated herein by reference, describes the preparation of ZSM-5 from reaction mixtures which are free of organic directing agents. However, the reaction mixture for making this organic-free form of ZSM-5 is restricted to silica to alumina molar ratios of 100 or less. Consequently, this organic-free synthesis tends to produce ZSM-5 having a relatively high acid catalytic activity (e.g. alpha value) in comparison with zeolites prepared by the method of the Dwyer et al. U.S. Pat. No. 3,941,871.
Co-pending U.S. application 140,360, filed Jan. 4, 1988, cited above and incorporated by reference as if set forth at length herein, disclosed improvement in selectivity toward valuable C.sub.2 -C.sub.4 olefins and C.sub.6 -C.sub.8 aromatics by reducing the alpha value of the composite zeolite catalyst. However, until the advent of the present invention, the criticality of maintaining the composite catalyst alpha value within a narrow range has not been appreciated.